Appendix A Description of the different sub-detector layers of the ATLAS detector I. The Inner detector The Inner Detector (ID) is situated closest to the beam pipe, and consists of three
sub-detectors; the Pixel detector, the Semi-Conductor Tracker (SCT) and the
Transition Radiation Tracker (TRT). It is surrounded by a superconducting
solenoid magnet which generates a 2 Tesla magnetic field. The ID measures the
curved tracks of charged particles which have been bent by the magnetic field
generated by the solenoid magnet. It is designed to make high precision
measurements, using the technologies of silicone s ensors and straw drift tubes.
Both the Pixel detector and SCT contain modules with silicone microstrips.
Charged particles passing through these modules remove electrons from the
silicone layer, which migrate towards solder spheres hence generating a current.
The current is read out by a layer of electronics and consequently digitized. Since
the solder spheres are distributed in an array across the modules, the trajectory
of the particle can be determined by tracking which spheres have a signal.
The TRT surrounds the SCT and pixel detectors. It is composed of ~50 000 gas
filled tubes, each containing a gold plated tungsten wire in the centre. As charged
particles traverse through the TRT, they ionise the gas and radiate photons which
further interact with the gas to free electrons. The gold wire acts as an electrode
to which electrons migrate and the current across the wire is measured. The TRT
can distinguish between different types of charged particles since electrons
generate more photons versus a pion and thus more negative charge is measured
across the wire. Since the charged particle paths are measured by the inner
detector under the influence of a known magnetic field, the momentum of these
